Chapter 3 Pulse Width Modulator

Transcription

1 Chapter 3 Pulse Width Modulator

2 Pulse Width Modulator 3-1: Curriculum Objectives 1. To implement the pulse width modulator by using µa To understand the characteristics and basic circuit of LM To implement the pulse width modulator by using LM To measure and analyze the pulse width modulation circuit. 3-2: Curriculum Theory Pulse width modulation (PWM) is a modulation method in between the digital and analog, which can be used to process the digital and analog data transmission. The amplitude of pulse width modulator is fixed, but the pulse width will be varied and controlled by the input audio signal amplitude. If we control the time variation of the electric level, then this means that we can control the width of the pulse. When the amplitude of the audio signal is getting larger, then the pulse width will become wide; on the' other hand, when the amplitude of the audio is getting smaller, then the pulse width will become narrow. Therefore, the PWM can be applied in the fast and slow of the rotation rate of the motor, the strong and weak of the light source of the light bulb and so on. The relationship between audio and pulse width modulation signal is shown in figure 3-1. Generally, pulse wave modulation can be classified as pulse amplitude modulation (PAM), pulse position modulation (PPM), pulse width modulation (PWM) and so on. Table 3-1 shows the comparison between each modulations and figure 3-2 shows the output characteristics diagram of PAM, PPM and PWM modulations. Figure 3-3 is a square wave oscillation circuit, the output signal pulse width is controlled by R 2, C 2 and V in (+) input terminal voltage. The op-amplifier μa741 is the comparator in this circuit. The V in (+) input (pin 3) reference voltage is decided by the resistor R 1 and variable resistor VR 1. R 2 and C 2 are constructed to become a charge/discharge path. When no signal supply to the audio signal input terminal, if we adjust VR 1, then the V in(+) input terminal operation voltage will

3 change, which means the reference voltage of comparator will change, thus, the output signal of pulse width will also change too. Figure 3-1 Signal waveforms of audio signal and PWM signal. Table 3-1 Comparisons between three different types of modulations. Modulation Types PAM PPM PWM Features Pulse amplitude will be varied with the amplitude of the input signal. Pulse position will be varied with the amplitudeof the input signal. Pulse width will bevaried with the amplitudeof the input signal. Pulse Amplitude Varied Constant Amplitude Pulse Width Constant Width Constant Width Pulse Interval Changeless Varied Constant Amplitude Varied Changeless

4 Figure 3-2 Output characteristics diagram of PAM, PPM and PWM modulations. Figure 3-3 Circuit diagram of PWM by using µa741.

5 Figure 3-4 Waveforms diagram of the charge and discharge of ua741. If the VR 1 is fixed, it means that the operation voltage of V in (+) input terminal is fixed. If input an audio signal to the audio signal input terminal, then the audio signal voltage will add to the operation voltage of the V in (+) input terminal. Besides, by following the path of charge and discharge of Rand C 2, the operation voltage of V in (-)will change as well, as shown in figure 3-4. However, when we change the bias point by tuning the variable resistor VR 1, we can change the level and the width of the output square wave of V in (+) and V in (-) at the same time. At this moment, the reference voltage of the comparator will be varied and controlled by the voltage of the audio signal. Therefore, the output signal of pulse width will also change with respect to the voltage of input audio signal,then the pulse width modulation signal is produced. The circuit diagram of LM555 astablemultivibrator is shown in figure 3-5. In figure 3-4, the circuit

6 can be divided into 5 important parts, which are lower comparator, upper comparator, flip-flop (FF), discharge transistor and output driver. If the controlled voltage terminal (pin 5) does not input any signal, then the upper comparator reference voltage is 2V CC /3 and the lower comparator reference voltage is V CC /3. If we add the controller voltage to the control voltage terminal (pin 5), the comparator reference voltage can be externally controlled. When the controlled voltage termingdoes not use, then we can make the controlled voltage terminal connects with a capacitor 0.01 μf to the ground to avoid the interference of noise. Figure 3-5 Circuit diagram of LM555 astablemultivibrator. Figure 3-5 is the circuit diagram of astablemultivibrator by using LM555 IC. The output signal of this circuit is a square wave. The oscillation frequency is determined by R 1, R 2 and C 1. The charge

7 time (t 1 ) of capacitor x(r 1 +R 2 ) x C 1 ; the discharge time (t 2 ) of capacitor C 1 is x R 2 xc 1. So the period T is the charge time t 1 plus the discharge time t 2 equals t o 0.693x( R 1 + 2xR 2 ) x C 1. Figure 3-6 sho ws t he o utput waveforms of LM555 astablemultivibrator at different points. Figure 3-7 is the circuit diagram of monostablemultivibrator by using LM555 IC. When the trigger input changes from high (+12 V) to low (0 V), the output terminal will produce a pulse. This pulse width T is determined by R 1 x C 1 actually is approximately 1.1 x R 1 x C 1. If R 1 = 10 kω and C 1 = 0.01 μf, then the pulse width is about 110 µs. If the frequency is less than 9.1 khz at the trigger signal input terminal (pin 2), (refer to the waveforms of astablemultivibrator in figure 3-6), then the output will be the 50 % duty cycle pulse signal. The audio signal is inputted by the controlled voltage terminal. Therefore, this will produce the PWM signal. Figure 3-8 is the circuit diagram of PWM by using two LM555 ICs, which U, which U 1 is the astablemultivibrator and U 2 is the monostablemultivibrator. By combining these two parts, we will obtain a PWM circuit. Monostablemultivibrator (U 2 )needs the trigger pulse from the astablemultivibrator (U 1 output terminal (pin 3), the audio signal is inputted at the controlled voltage (pin 5) of the monostablemultivibrator (U 2 ). The PWM signal is outputted at the output terminal (pin 3) of the monostablemultivibrator.

8 Figure 3-6 Output waveforms of LM555 astablemultivibrator at different points.

9 Figure 3-7 Circuit diagram of monostablemultivibrator by using LM555 IC. Figure 3-8 Circuit diagram of PAM by using two LM555 ICs.

10 3-3: Experiment Items Experiment 1: µa741 pulse width modulator 1. Refer to figure 3-3 or figure DCT3-1 on GOTT DCT module, let J1 be open circuit, that means R 1 is unused. 2. Adjust the variable resistor VR 1 so that V in (+) input terminal voltage is 0 V. Then let J1 be short circuit, that means let R 1 is used. 3. At the audio signal frequency input terminal (Audio I/P), input a 3 V amplitude and 500 Hz frequency waveform. 4. By using oscilloscope, observe on the signal waveform of audio signal. Input terminal and output terminal (pin 6). Then record the measured results in table Let J1 be open circuit, then record the audio input signal. Adjust VR 1 so that V in (+) voltage of input terminal is 6 V. 6. Let J1 be short circuit, that means let R 1 is used. Then input the audio signal terminal into the original audio signal. 7. By using oscilloscope, observe on the signal waveforms of the audio signal input terminal and output terminal (pin 6). Then record the measured results in table Let J1 be open circuit, that means R 1 is unused and record the audio input signal. Adjust VR 1 so that V in (+) voltage of input terminal is -6V. 9. Let J1 be short circuit, then input the original audio signal into the audio signal input terminal. 10. By using oscilloscope, observe on the signal waveforms of the audio signal input terminal and output terminal (pin 6). Then record the measured results in table Let J1 be open circuit and record the audio input signal. Adjust VR 1 so that V in (+) voltage of the input terminal is 0 V, then let J1 be open circuit. 12. Change the audio signal amplitude to 5 V, the others remain the same. Repeat step 4 to step 10 then record the measured results in table 3-3.

11 Experiment 2: LM555 pulse width modulator 1. Refer to figure 3-8 or figure DCT3-2 on GOTT DCT module. 2. By using oscilloscope, observe on the test point TP3 and the output Signal waveform, at the same time adjust variable resistor VR 1 until when the square wave signal of test point TP3 at differences voltage level, the output square wave signal has different pulse width. (i.e. different duty-cycle). 3. At the audio signal input terminal (Audio I/P), input a 2.5 V amplitude and 1 khz frequency square wave. Then record the measured results in table By using oscilloscope, observe on the output signal waveforms of the discharge capacitor TP1, critical point TP2, trigger signal TP3, critical point of the discharge capacitor TP4, and PWM O/P. 5. By using oscilloscope and switching to DC channel, observe on the output signal waveforms and record the measured results in table Change the input signal to triangular wave, the others remain the same, repeat step Change the input signal to sinusoidal wave, the others remain the same, repeat step Change the input signal amplitude to 1.5 V, the others remain the same, repeat step 6 to step 7, then record the measured results in table Repeat step 3 to step 5, then record the measured results in table 3-7.

12 3-4: Measured Result Table 3-2 Measured results of µa741 pulse width modulator. (V m = 3V, f m = 500 Hz) DC Bias Voltage at Input Signal Waveforms V in (+) Output Signal Waveforms 0V 6V -6V

13 Table 3-3 Measured results of μa741 pulse width modulator. (V m =5 V in, f m = 500 Hz) DC Bias Voltage at V in (+) Input Signal Waveforms Output Signal Waveforms 0V 6V -6V

14 Table 3-4 Measured results of LM555 pulse width modulator. (V m = 2.5 V, f m = I khz) Input Signals Input Signal Waveforms Square Wave Triangular wave Sinusoidal Wave

15 Table 3-5 Measured results of LM555 pulse width modulator. (V m = 2.5 V, f m =1 khz Square Wave) Input Signals Input Signal Waveforms Square Wave Triangular Wave Sinusoidal Wave

16 Table 3-5 Measured results of LM555 pulse width modulator. (V m = 2.5 V, f m =1 khz Triangular Wave) Test Points Output Signal Waveforms TP1 TP2 TP3 TP4 PWM O/P

17 Figure 3-5 Measured results of LM555 pulse width modulator. (Continue) (V m = 2.5 V, f m =1 khz Sinusoidal Wave) Test Points Output Signal Waveforms TP1 TP2 TP3 TP4 PWM O/P

18 Table 3-6 Measured results of LM555 pulse width modulator. (V m = 1.5 V, f m = 1kHz) Input Signals Input Signal Waveforms Square Wave Triangular Wave Sinusoidal Wave

19 Table 3-7 Measured results of LM555 pulse width modulator. (V m =1.5 V, fm = 1 khz Square Wave) Test Points Output Signal Waveforms TP1 TP2 TP3 TP4 PWM O/P

20 Table 3-7 Measured results of LM555 pulse width modulator. (Continue) (V m =1.5 V, f m =1 khz Triangular Wave) Test Points Output Signal Waveforms TP1 TP2 TP3 TP4 PWM O/P

21 3-5: Problems Discussion 1. What are the functions of VR 1 in figure 3-3 and figure 3-8? 2. If we change the capacitor C 6 to 0.1 µf in figure 3-8, the others remain the same, does the output still show the PWM waveform? Why? 3. For output voltage polarity, what are the differences of PWM signal between experiment 1 and experiment 2?